Radio transmission systems of point-multipoint type are known to the person skilled in the art by the initials MMDS (standing for Microwave Multipoint Distribution System), LMDS (standing for Local Multipoint Distribution System) and MVDS (standing for Multipoint Video Distribution System). These systems used for the broadcasting of programmes permit a return link for the subscriber terminals which allows the subscriber to interact with the programme received.
In Europe, provision has been made to implement a system of LMDS type which has 24 broadcasting channels (also referred to as downlinks) having a bandwidth of 33 MHz, and 25 return channels (or uplinks) having a bandwidth of 2 MHz, these channels being situated between 40.5 and 42.5 GHz (for further details on the apportioning of the channels, the person skilled in the art may consult the MPT-1560-RA standard). The system implemented must comply with the ETSI standard 301199 better known as DVB LMDS which among other things makes provision for an oscillator drift of plus or minus 200 kHz for the uplink, the drift being due mainly to climatic conditions. For further information about the systems, the person skilled in the art may refer for example to the patent application WO 2002/33855.
The bandwidth allotted for this type of application has been increased and currently corresponds to a frequency band lying between 40.5 and 45.5 GHz. Provision is also made to segment this band so as to apportion it among several operators.
FIG. 1 illustrates an exemplary distribution network for an operator deploying systems of this type. Base stations ST are deployed over a given zone. A base station ST covers one to four sectors A, B, C and D of finite size. Each sector uses a different frequency band and/or polarization so as to reduce the problems of interference between adjacent sectors.
FIG. 2 shows a configuration where a first operator has deployed three base stations ST and where a second operator has deployed his own base station ST′. A more considerable deployment of the two operators does not pose any problem of interference insofar as the frequencies allotted to the operators are completely different.
Such systems compete with other distribution networks such as satellite, cable or telephone. However, these systems may be used by a cable operator who wishes to extend his network without having to fully cable a given zone. For this purpose, operators desire that the decoders placed at subscribers premises be the same as for cable so as to avoid development overhead costs. Furthermore, it is desired that this type of network be able to serve as wireless bridge for a cable network.
FIG. 3 illustrates a distribution network system serving as relay for a cable network. A base station ST furnished with a transmitter, and possibly with a receiver, broadcasts information destined for a plurality of subscribers. On the subscriber side, an external unit 1 is connected to a cable network 2. A subscriber can connect up to the cable network 2 with the aid of an internal unit 3 which serves as interface to one or more user appliances 4. The external unit comprises an antenna and means for transposing the signals received into a frequency band compatible with the cable network 2 and means for transposing signals to be transmitted to the base station ST. The internal unit 3 is for example a TV decoder or a modem intended for the cable network 2. The user appliance 5 is for example a television, a telephone or a computer.
The production of an external unit requires compliance with transmission conditions related on the one hand to the radio path and on the other hand to the cable path. The DOCSIS standard which relates to cable networks permits a frequency drift of 32 kHz for the uplink and 30 kHz for the downlink. If this frequency drift is taken back to the radio frequency used, it is noted that the stability of the oscillators situated in the external unit 1 must be less than 1 in 1 million. But the external unit 1 is subjected to temperature and climate constraints which make it difficult and hence very expensive to achieve such stability.
A known solution consists in using a reference subcarrier sent by the base station so as to synchronize the external unit. Such a solution is described for example in U.S. Pat. No. 5,844,939. FIG. 4 represents a circuit corresponding to the type of circuit disclosed by the said patent. The signals received are transposed by a mixer 5. From among the signals transposed, a filter 6 selects a portion of band in which only the reference subcarrier should be situated. A phase/frequency comparator 7 compares the filtered frequency with a reference oscillator 8 and operates an oscillator 9 which provides the transposition frequency to the mixer 5. The frequency feedback control thus carried out corresponds to a phase-locked loop which makes it possible to achieve an absolute error in the frequency of the oscillator 9 which is equal to the error in the reference oscillator 8. Additionally, this technique compensates furthermore for the frequency drift of the base station. Moreover, if a return link uses one and the same oscillator to transpose the uplink and downlink, the error in the return link is also compensated.
Such a system may not be used in a cellular distribution network corresponding to FIGS. 1 and 2. The use of a fixed frequency for the entire distribution network would cause interference between the zones using one and the same frequency.